—
The occurrence of turbulent pulsations in straight pipes of noncircular cross-section leads to the situation, when the average velocity field includes not only the longitudinal component but also ...transverse components that form a secondary flow. This hydrodynamic phenomenon discovered at the twenties of the last century (J. Nikuradse, L. Prandtl) has been the object of active research to the present day. The intensity of the turbulent secondary flows is not high; usually, it is not greater than 2–3% of the characteristic flow velocity. Nevertheless, their contribution to the processes of transverse transfer of momentum and heat is comparable to that of turbulent pulsations. In this paper, a review of experimental, theoretical, and numerical studies of secondary flows in straight pipes and channels is given. Emphasis is placed on the issues of revealing the physical mechanisms of secondary flow formation and developing the models of the apriori assessment of their forms. The specific features of the secondary flow development in open channels and channels with inhomogeneously rough walls are touched upon. The approaches of semiempirical simulation of turbulent flows in the presence of secondary flows are discussed.
The stability and nonlinear interaction between the disturbances in a round jet are investigated numerically at Re = 2850. The conditions of the laboratory experiment performed earlier in the ...Institute of Mechanics of Moscow State University are reproduced. The characteristic feature of the jet considered is the presence of three points of inflection in the inflow velocity profile. This fact determines essentially the properties of flow. Linear stability is investigated using two approaches: quasi-parallel and spatial. Good quantitative agreement between the results of two approaches and adequate agreement with the results of inviscid theory are obtained. Numerical calculations are carried out with the aim to explain and interpret the results of the laboratory experiment in which change in the length of the laminar-turbulent transition zone in the jet under the action of time-periodic axisymmetric perturbations is revealed. It is shown that axisymmetric disturbances of even considerable initial amplitude do not lead to laminar-turbulent transition. The transition observed experimentally is attributable to the presence of non-controlled three-dimensional disturbances that are strengthened against the background of fairly intense artificial disturbances. The investigations carried out in the present study confirm this hypothesis. Thus, the three-dimensional disturbances serve as an initiator of transition, while the axisymmetric disturbances of a fairly high amplitude only accelerate their growth. It is shown that more intense three-dimensional disturbances are able to ensure still more rapid transition even in the absence of axisymmetric component.
Pulsating turbulent flows in a square pipe are studied numerically. The flow dominance regime in which the fluid flow rate remains positive in all phases of the oscillatory cycle is considered. The ...flows are studied at several oscillation frequencies. The results are compared with oscillating laminar flows and a steady turbulent flow in a square pipe, as well as with pulsating turbulent flows in a round pipe. The integral and fluctuating characteristics of turbulence and their dependence on the oscillation frequency are determined. In particular, it is found that at the considered Reynolds number Re = 2200 the friction coefficient in pulsating flows turns out to be lower than that in the stationary flows. The drag reduction increases with growth of the oscillation period and reaches 14.7%. A distinctive feature of turbulent flows in pipes of rectangular cross-section is the occurrence of secondary flows of Prandtl’s 2nd kind. The details of secondary flows under the pulsating flow conditions are studied at length.
In the classical experiments of O. Reynolds made in 1883 the critical value of a dimensionless parameter, named now the Reynolds number,
, was determined. As this value is exceeded in a pipe of ...circular cross-section, a turbulent flow regime can occur. The attempts to define this value more exactly undertaken during the twentieth century have not met with success. In this study, we present a review of theoretical, experimental, and numerical investigations of flows in a round pipe at the stage of transition to turbulence performed in recent years, which make it possible to formulate a new view on the nature of laminar-turbulent transition in these flows.
To explore the direction of inter-scale transfer of scalar variance between subgrid scale (SGS) and resolved scalar fields, direct numerical simulation data obtained earlier from two ...complex-chemistry lean hydrogen–air flames are analysed by applying Helmholtz–Hodge decomposition (HHD) to the simulated velocity fields. Computed results show backscatter of scalar (combustion progress variable $c$) variance, i.e. its transfer from SGS to resolved scales, even in a highly turbulent flame characterized by a unity-order Damköhler number and a ratio of Kolmogorov length scale to thermal laminar flame thickness as low as 0.05. Analysis of scalar fluxes associated with the solenoidal and potential velocity fields yielded by HHD shows that the documented backscatter stems primarily from the potential velocity perturbations generated due to dilatation in instantaneous local flames, with the backscatter being substantially promoted by a close alignment of the spatial gradient of mean scalar progress variable and the potential-velocity contribution to the local SGS scalar flux. The alignment is associated with the fact that combustion-induced thermal expansion increases local velocity in the direction of $\boldsymbol {\nabla } c$. These results call for development of SGS models capable of predicting backscatter of scalar variance in turbulent flames in large eddy simulations.
•We studied the hemispheric asymmetry of oscillatory activity evoked by moving sounds.•The spectral perturbation (ERSP) and inter-trial phase coherence (ITC) were measured.•The spectral perturbations ...were stronger in the right hemisphere.•The delta-alpha ITC showed stronger contralaterality in the left hemisphere.•The asymmetry of motion-onset response was similar to that of ITC (“neglect model”).
The present study investigates hemispheric asymmetry of the ERPs and low-frequency oscillatory responses evoked in both hemispheres of the brain by the sound stimuli with delayed onset of motion. EEG was recorded for three patterns of sound motion produced by changes in interaural time differences. Event-related spectral perturbation (ERSP) and inter-trial phase coherence (ITC) were computed from the time-frequency decomposition of EEG signals. The participants either read books of their choice (passive listening) or indicated the sound trajectories perceived using a graphic tablet (active listening). Our goal was to find out whether the lateralization of the motion-onset response (MOR) and oscillatory responses to sound motion were more consistent with the right-hemispheric dominance, contralateral or neglect model of interhemispheric asymmetry.
Apparent dominance of the right hemisphere was found only in the ERSP responses. Stronger contralaterality of the left hemisphere corresponding to the “neglect model” of asymmetry was shown by the MOR components and by the phase coherence of the delta-alpha oscillations. Velocity and attention did not change consistently the interhemispheric asymmetry of both the MOR and the oscillatory responses. Our findings demonstrate how the lateralization pattern shown by the MOR potential was interrelated with that of the motion-related single-trial measures.
The classicality concept is formulated. It is used to derive Wigner inequalities dependent on three instants. The possibility of testing experimentally violations of these inequalities in pure and ...mixed flavor states of a pair of neutral pseudoscalar mesons is studied. Within the Werner background model, it is shown that a violation of time-dependent Wigner inequalities may be observed even at a 50
fraction of background processes.
Partial widths and double- and single-differential distributions are calculated on the framework of the Standard Model for the four-lepton decays
, where
=
and
. The contribution of virtual-photon ...emission by a light
quark and a heavy
quark, the bremsstrahlung contribution from charged leptons in the final state, the weak-annihilation contribution, and the contribution of wide vector
resonances are taken into account. The contribution of narrow
and
resonances is excluded. Virtual-photon emission by the light quark of a
meson is described on the framework of the vector-meson-dominance model, within which only the contributions of the lightest vector mesons containing
pairs are taken into account. For the
meson, this is the
resonance. For the
meson, the contributions of the
and
resonances are taken into account. Two possibilities are considered in the present study. The first is that where the contributions of the narrow
and
mesons are excluded from the partial widths with respect to the corresponding decays. The second is that where the contributions of narrow resonances are taken into account in partial widths. The problem of the influence of the subtraction procedure on the partial widths with respect to the decays
is studied individually.
A novel method to extract individual free-radical polymerization rate coefficients for butyl acrylate intramolecular chain transfer (backbiting), k bb, and for monomer addition to the resulting ...midchain radical, , is presented. The approach for measuring k bb does not require knowledge of any other rate coefficient. Only the dispersion parameter of SEC broadening has to be determined by fitting measured MWDs or should be available from separate experiments. The method is based upon analysis of the shift in the position of the inflection point of polymer molecular weight distributions produced by a series of pulsed-laser polymerization (PLP) experiments with varying laser pulse repetition rate. The coefficient k bb is determined from the onset of the sharp decrease of the apparent propagation rate coefficient ( ) with decreasing repetition rate, an approach verified by simulation. With experiments performed between −10 and +30 °C, the estimated values are fitted well by an Arrhenius relation with pre-exponential factor A(k bb) = (4.84 ± 0.29) × 107 s-1 and activation energy E a(k bb) = (31.7 ± 2.5) kJ·mol-1. At low pulse repetition rates, the experimental values are related to an averaged propagation rate coefficient, , that is dependent on the relative population of chain-end and midchain radicals. Evaluated by comparing simulated and experimental molecular weight distributions, provides an estimate for . The Arrhenius parameters are: A( ) = (1.52 ± 0.14) × 106 L·mol-1·s-1 and E a( ) = (28.9 ± 3.2) kJ·mol-1.
Abstract
In the framework of the Standard Model we present new theoretical predictions for the branching ratios, double and single differential distributions and forward—backward leptonic asymmetries ...for the
B
¯
s
→
μ
+
μ
−
e
+
e
−
decay. In our consideration we take into account the
ϕ
(
1020
)
—resonance contribution; the main contributions of four charmonium resonances :
ψ
(
3770
)
,
ψ
(
4040
)
,
ψ
(
4160
)
and
ψ
(
4415
)
;
u
u
¯
—resonant contribution from
ρ
(
770
)
and
ω
(
782
)
;
‘tails’ contributions from
J
/
ψ
and
ψ
(
2
S
)
resonances; non—resonant contribution of the
b
b
¯
- pairs, bremsstrahlung and the contribution of the weak annihilation. We provide the prediction for the branching ratio of
B
¯
s
→
μ
+
μ
−
e
+
e
−
decay with and without the
ϕ
(
1020
)
—resonance contribution. We use the model of vector meson dominance (VMD) for calculation of resonances contributions and take into account all substantive terms in
B
¯
s
→
μ
+
μ
−
e
+
e
−
amplitude that was not considered in the previously papers.
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